Sometimes the optimum flow conditions for a fluid passing through a heat exchanger can not be determined until after the heat exchanger is put into operation. For example, when concentrating radioactive waste liquors which have a predominant scaling tendency, it is advantageous to heat such liquors at as high a velocity as is practical. But the optimum velocity for minimum power consumption consistent with maintaining scale-free operation is often unknown. In addition, future operations may include crystallization of salts from the concentrated liquors, and it may be impossible to predetermine the optimum velocity for minimizing erosion and extending tube life when operating as a crystallizer. Thus it is desirable to be able to change the velocity of the fluid flowing through the heat exchanger after the exchanger has been installed and operated. This can most easily be accomplished by changing the number of passes the fluid being heated makes through the tubes of the exchanger. Such flexibility was not easily attained with prior art apparatus in that extensive and costly modification of the equipment or excessive down time was required.